Multioffice telegraph system



.' 11, 1945. F. CURRIE ET AL MULTIOFFICE TELEGRAPH SYSTEM 5 Sheets-Sheet 1 Filed Aug. 2, 1940 INVENTORS v F.L.C|URR|E jiisrzmms ATT RNEY .TO FIG.4 TO FIG. 4

D 1945- F. L. CURRIE ET AL 2,390,517

MULTIOFFI CE TELEGRAPH SYSTEM Filed Aug. 2, 1940 5 Sheets-Sheet 3 TO FIG 5 To FIG. .59

INVENTORS F. L.CURRI E BY Egg ATTO NEY FIG. 3

Dec. 11, 1945. F. L. CURRIE ET AL 2,390,517

MULTIOFFICE TELEGRAPH SYSTEM Filed Aug. 2, 1940 5 Sheets-Sheet 4 I INVENTORS F.L.CURR|E C.F.STEARNS o o .lolo .lolo o o o I 0 0 o o 0 o o y o o o o lo o o w 'ro Fl G.3

. 11, 1945. F. CURRIE ET AL MULTIOFFICE TELEGRAPH SYSTEM Filed Aug. 2, 1940 5 Sheets-Sheet 5 m SE RRA Y ORE E U.T -N H R L T NFC M cuit to hold relay l4 energized and to hold the armature of polar relay 42 in engagement with its left hand contact. The armature of relay 42 is biased so that when the polarity of the line circuit 20 is reversed or when nc current flows through its 'iwinding, jsaid armature will engage the right hand contact of said relay.

When the key I2 is closed as described above, a.

'g 1 s eam circuit is closed through the Winding of relay I5," the contacts of key I2 and the left'hand armature and front contact of relay I 4 for energizing relay I5. The energization of relay I closes through its inner right hand preliminary'make contacts a holding circuit for relay I4, and at its right hand outer armature transfers the line circuit from the winding of relay 'I4' through the 01f normal contacts of the dial I3 and the operating,

magnet of the printer I0 to negative battery, thus causing a reversal of the polarity of the line conductor'20.'.;"A. circuit-isalso closed through. the -left.-hand"-armature and front contact'ofv relay I5 tolight the fbusy3lamp.I6.. g 5 Upon the reversalof the potentialapplied to thetrunk theline 'ccurrent is reduced because of therectifie'r 46 inseries with the line circuit, and thearmature of relay 42 moves into engagement. with itsright'hand contact, thereby closing thecircuit of slow release relay 4'! and opening .theflcircu'it of slow. release relay 48. The contacts -.of. relay 42- are connected, as shown, to the operatingmagnetsof the selector'switch 50 and the minor switches 5I 'and,52 to directively control said switches in :response to the operation of the dial .I3i-in'substantial1y the same manner as disclosed in the above-mentioned Blanton patent. When the. dialiI3 is turned, the off-normal contacts I 9 open and remain open until the dial has .-returned.-'to its, normal position. When the dial is released, the dialing contacts transmit a series of positive impulses over the line circuit to operate the' armature of relay 42 into engagement withitszleft hand contact, one or more. times in accordance with the number dialed. When the line circuit was initially opened, the release of re- I lay. closed the circuit of-the rotarymagnet of the minorswitch 5|, thereby stepping the wiper "of; said switchinto engagement with its second contactconnected to the, rotary magnet of the.

hundred gpoint selector switch 50; may be em-,

ployed' to select: more than: one hundred and as 2 many as two hundred branch oflices or stations.

.At the end of the first digit otthe number dialed,-the,rele.ase ,of relay 48. which was operated .by;.the first dialing impulse, recloses the circuit ,of therotary magnet ofthe minor switch 5| and .thewiperof t e said switch is step d into enagement withits third contact, thereby closing the ,circuitthrough the rotary magnet of the selegctor switch 50. At; the end of the second digit,

the wiperof the minor switch 5I takes. another step to close the circuit to the vertical, magnet .ofthe selector switch 50, and this magnet .re- .sponds to the third-digit dialed. As soon as the selector switchf50 takes .its first step, the oft-,

are associated with-the branch oifice printer 30, Fig. 2... It will be noted thatafter thelast digit said line'circuit through the winding of said-relay the left hand inner, armature of relay 62. branch ofiice a polar relay; I0 and, resistance II normal contacts are closed, but the slow re lease relay 4! holds its contacts open during the dialing operation and thus holds open the circuits of the release magnets through the off-normal contacts 55- of the selector switch and the off-normal contacts 56 and 51 of the minor switches 5 I and 52 until afterthe callis released.

The oiT-normaI-contacts 58 of the minor switch "5I' close "as soon as the selector switch is seized ,and connect ground through the conductor 59 for energizing the relay 6| which serves to prevent the answering equipment associated with the trunk 20 from answering any succeeding call from a branch oi lice while the trunk is in use, in a j manner to be described below. It will be apparentthat this'is necessary since the trunk 20 is already busy as it has been seized by an operator at the main o'fficefor the purpose of extending a call to a wanted branch oflice.

'The detailed operation of the selector switchand the way in which the busy 'oridle condition of the called line is tested. and-the connection made if it is idle are substantially. the same-as described in the above-mentioned Blanton patent. Reference is therefore made to said patentfor a fuller description of these .andzother'details'not fully described herein. Y It may be assumed that the wipers 53 and 53a of the selector switchare stepped by the operation of :the dial; I31 into engagement as shown'with the bank contacts-:which saidrelays become energized.

Each branch oflice onstation hasass ociated therewith-two control relays corresponding to the relays 62 and 63 individual to the line circuit 32 extending to the selected office. As shown in s Fig. 2,;a pair of'similar, relays 62a and 63a are 1 associated in a similar way with theline circuit 33 to a secondbrancnofiice. Since the circuits and control equipment of allof the outlying stations aresimilar, the operation of the system will be clear from a description of the steps involved in extending calls to and from one station. Relay 63 is normally energized in series with the line circuit ,32, positive battery beingv applied to and the normally Tclosedcontacts associated with At the are-connected in series relationwith. the printer magnet, the keyboard contacts. of the printer.- 30

and the line circuit; When positivebattery is" applied to the. line circuit 32 at the switching oifice .as, described; current flowing throughv the winding of ,polarrelay 10 holds the contacts of saidrelay open.

When, the selector. switch wipers 53 and 53a engage the terminals of the called branch ofiice,

a circuit is closed from ground through resista. ance 49, the Winding of marginal relay 43,'th left hand armatureand front contact of relay 45,-the right hand wiper of. the minor switch 52 which 62 to battery whereupon relay.62. becomesenergized. The energizati-on of relay 62L interrupts the above-described circuit ofrelay. 6 3. but mainisms s i re -at a e-ad area l f .the main office trunk 29 is idle.

of said relay, conductor 9|, resistance 92, the

.winding of. relay '93,.the interrupter contacts and stepping magnet of the switch 8 I, 'the right hand outer. armature and back contact of relay 4| and the left hand armature andback contact of relay gized. The energization'of relay. 90 further'removes ground applied through its armature and back contact .and the conductor 95 to a bank contact, in this instance the first bank contact-associated. with'the' first switchwiper or the group switches 80, 80a and 80b, whereby the group control switch associated with the first idle trunk is able to select the particular finder switch, such as 16 or 16a of the main office trunk controlling the group of lines in which the call has been initiated.

The switch wipers of the allotter switch 8| are in engagement with their first bank contacts if As the several trunks to the main o-flice are seized for extending calls between the branch offices and the main office, the wipers of the switch 8| are stepped to directsucceeding calls to idle trunks in a manner "to be described. Assuming that the trunk 20 is idle, a circuit is closed upon the energization of relay 93 from battery through the right hand armature and front contact of said relay, the

- third switch wiper of switch 8| and its first bank contact, the fifth right hand armature and back contact of relay 4| andthe winding of relay 96 to ground, whereupon relay 96 becomes energized. Upon the energization of relay .96, a circuit is closed from battery through the outer armature and front contact of relay 96, theinterrupter contacts and stepping'magnet of the switch 80 and the first wiper of said switch to ground through the contacts of relays corresponding to relay 90 associated with the groups of lines in which the particular calling line is not located. When the ywipers of the group control switch 80 reach the position shown in whichthere is no ground on the bank contact engaged by the first wiper of said switch, relay 9'! becomes energized through the innerarmature and front contact of relay 96 'in'series relationwith the stepping magnet of said switch, but the current traversing the stepping magnet is reduced to such a valu that the switch is not stepped.

.Upon the energization of relay 9! a circuit is completed for operating the finder switch 76 to start hunting the trunk contacts of the calling branch office if.the switch wipers are not already engaging said contacts. As already described, thedeenergization of relay 63' removes ground from the first bank contacts associated with the first switch wipers of switches lfi, ll, etcetera.

If the switch wipers of switch 16 had been in engagement with other bank contacts, ground on said contacts connected through the first wiper of said/switch, conductor 99; the third switch wiper of the switch 80 and it first bank contact, second right hand armature and back contact of relay 4|, the inner armature and front contact of relay 91, second switch wiper; of switch 80 and .(il .to battery whereupon relay 93 becomes enerconnected to the trunk 20 at the main changed frompositive to negative.

The ener'gizationof relay 4| further closesa relay 91,. and the winding .of relay 4| toground, whereupon relay 4| "becomes energized and through its second right hand armature and front contact and conductor 99'applies ground potential to the first bank contacts of. the first levels of switche 16, 11, et ceterato prevent any Other finder switch from seizing the terminals 'of .the line which has been selectedby .the finder switch 16. The energization of .relay 4| interrupts at i ts fifth' right hand armature and back contact th circuit of relay 96 but relay 9! remainslocked up through a locking circuit including the inner right hand armature and front contact of relay, the middle armature and back contact of relay I03, and the outer armature and front contact of relay 9'! to positive battery. The energization of relay 4| also closes a circuit through its fourth right hand armature and front contact from ground through the winding'of slow-to-operate relay I04, whereupon relay I04 becomes energized. The energization of relay 4| further interrupt momentarily the line circuit or trunk 20. to the main ofiice at the inner left hand armature and back contact of said relay thereby releasing the normally energized relay H at the main ofiice to initiate a call to said office. i

When relay |4 deenergizes, a circuit is closed from ground through the left hand armature and back contact of said relay-for lighting the supervisory lamp l1. Relay l4 further extends the trunk circuit 20 through the right hand armature and back contact thereof and the contacts and operating magnet of the printer III to negative battery so that the printer is operatively connected in circuit and the polarity of the source of curren circuit from ground through the third right hand armature and front contact of said relay, the fourth switch wiper of switch 80 in engagement with its first bank contact, conductor )6, second switch wiper of switch 16 in engagement with its first bank contact, the conductor I0! and the winding of relay 62 to battery, whereupon relay 62 becomesenergized. The energization of relay 62 disconnects 'the branch ofice line circuit 32 from the winding of relay 63, and opens the short-circuit around the windingof relay 63 whereupon said relay becomes energized through a circuit including the'left hand inner armature and makebefore-break contact of relay '62, thereby opening the start circuit for the line finder equipment at the outer armature :and back contact of relay63. Relay 90 becomes deenergized and applies-ground potential through the conductor 95 to the line terminals of the first level of group control switches 80,.80a and 80b. The deenergization of relay 90 further opens at its front contact the circuit of relay 93 and relay 93 becomes deenergized; Relay 93 mayv also be released by the operation of relay |||4 as described, since ground potential is connected through the left hand outer armature, and frontcontact of relay I94 to the left hand terminal of the winding of relay 93.

- Whenthe polarity of the main oflice trunk 20 V is reversed by the deenergization of relay I4 as above described, the contact of the armatureof polar relay I09 having its winding in series with the line circuitis actuatedinto engagement with its right hand contact, thereby closing a circuit from ground through said contact, the second left hand armature and front contact of relay 4| and the winding of relay )3 to battery, whereupon "relay 13 becomes energized. The energization of re iar W3 shorhclrcuits-ithe jwindingflif polar *remy "109 throi-rgh-its outer armature'andfront-con- "tact and the rectifier l-FZffind closes aholdirrg circuit for relay 4| wean-ma betraced through the inner armature and front contact-'0? relay i=0 3, "second =wipor-of-switeh8fl in engagement'with its first bank =ecntact',"conductor ltcandth-eon crating magnet andinterrupter-eontacts of switch "18 topcsitive batt'ery. The resistance of this winding ofrelay fl is sufficiently large to prevent ewi'tch- 1 6 from "Stepping. It will be noted thatrel'ayd I- connects-positive battery through its inner left 'hand armature andfront contact to'the-winddream rela-yfl 1 thushirlding its armaturein engagement with'its left hand-contact and-maintaining the branch o'fll-ee selector "50 and associated equipmentin the condition. The main dfllce tnx'nk 10 is'- now connectedthrough=*the=rectifier t [1, the "outer armature "and f-ront contact of relay I83, the left inner armature and "front contact or relay F04, the "fifth wiper'nf' control switch 80 in engagement with first bank contact, conductor H 0, the th-ir'd wiper of the finders-witch lt in engagement with its first bank con-tact, and the conductor l H to the branch omcewrunk M *so 'that the-printers 4*0 and 'iio "at themain and branchofllces;respectively, are interconnected for two-way communication.- lhe operator at the main o'fiice notes the lighting of thelamp t l and transmits GA tgo aheadl-"t-o at) 'the branch oflice to indicate that the-call has been switched 'through=and 'the branch offi'ce 'operator may commence to senti. l l

At the end of the call the operator at themain oilice' presses :thepush'lbutton or release key l8 whereupon: a circuit is closed through the: contacts of said hey and the winding of relay 1M :fornenergizing said any iRelay EM becomes locked up in series relation with the lineecircuitflfl and :positive'i battery, ithusireversing :the polarity oizsaili 14:!) trunk, and opens =the circuit of :the supervisory lamp 1 1 The l reversaliof .ithe polarity l of lthe iline orotrunkcircuit Z0, energizes the :windingcotrelay ma since i said winding 2 is :not short-.circuited thy the :circuit including: themectifier a li litter ithis TDD- larlty; and the armature of 1 said ireley'is; actuated into engagement with Sits left hand "contact "to open the circuit through :itsrright. ihan'd contact and the winding "of; relay Ma Relay; Ill-3i becomes deenergizecl and hpens thew-holding: circuitaof :retayd I -.whereup.on said .xelay released 'and \thc finder switch.eontroleequipmmtliszrestored toethe idle condition. Theeopening ofthe .cireuitsofsrrelaysl. 6.2 .sand I Mgatthathitd-find .fourth right. hand arm'atures; and "front contacts 10f "relay M 1 causes said. relays tofbe deenengized. Upon the .deene'rg'lzation; ofrrelay: 14 {17118 maimofllce trunk lfl may hezseized: for; a subsequent coal] under the control 0'1lhexallottervswitchrfi l The release of rrelay '62 IGC-IOSGS'QthB line: circuit. :3 2.:from1the -branch-ofiice in -series relation :with Fzthe normally, energized.:relay :63, rendering ttheibranch :o'flice available a for came through :the switching mentor. 3 "The allotter switch ill is providedWithdriving circuits as indicatdincludirrg:c'ontacts. of the .re-

specttve slow operate relays rzl 04 individual to the respective m a in omce trunks-wherebythe allotter switch invafiably stepstothe "contacts representingthe lowest nuniberedmain oflice trunk. When the respective trunks are busy,=l.the= control'relays I 04, 104a, l net cetera, areenergized and ch cuits are'closed-as'shownfrom the ground through the "outer I left hand armature and front "contact of relay 1'04 and the outer "right hand *armatures and front contacts or relaysfiMm mama to the 1 corresponding bank contacts of "the "first level-of 'theallotter switch BI, and the'lower interrupter contacts and the stepping magnet of saidswitbh forenergizing said stepping-magnetwherebythe wipers of the switch step-over the bank contacts of busytrunks. h 1 In the "case "of the combined "calling and answering trunks, such 2'8, the relays I I operate asdescribed' above when the trunk is seizedatthe main oflice, and a circuit *is closed for stepping the allotter switch 8! 'as'shown atthe inner armature and' front contact of therelay 6 I individual to the trunk "it. -On theother hand, if the wipers of the allotter switch 8 I are resting upon bank contacts ofanidle trunk higher than the first trunk and i the" first trunk becomes idle, driving circuitsare closed forthestepping magnet of switch tostep the Wipers over both busy and idle trunks back'to' the first trunk. Sirn'ilarly,if the switch wipers are ineneagernent with'a trunk higher than thesecondftrunk, and the-second trunk becomes idle, the switchwipers will be stepped into engagementwith said secondtrunk if the first'trunk'is busy. Assume for-example that relay lM-is energized indicating that the'first trunk is busy, that the switch wipers ofthe switch 8| are in engagement with theirsecondbank contacts representing the second trunk and the first "is released bythe main office operator so that it is available for the'next call. l Relay IM'becomes deene'rgized When the truri'li circuit is released whereupon a circuit {is closed from ground through'the right hand armature and -back contact of relay J04, the right hand outer armature and "back contact "o'f'tl; the second wiper of switch"8l in engagement'with its'second bank contact, lefthand armature-and back contact of relay B3, stepping magnet and lower interrupter contacts or switch "8|, right hand outer armature and back contact ct -relay 4|, and the'left'hand armature and back contact of relay 6| to battery. The contactdfrelaytil just referred to prevents stepping of the allotter switch incase the "first trunk is released after relay 93 hasoperated to cause an-idle trunk to be "seized; "It'will be'apparent thatas the irst wiper ofthe allotterswitch engages its bank con tacts corresponding to busy trunks, the circuit of the stepping-magnet of said switch islosecl through the respective bank contacts of the fi-rst level of'the switch to ground at'the armatures and front contacts of relays 1'04, 1 04a, 111412, -et cetera. 'In this manner the allotter switchf is stepped over the busy trunks. Assume, however, that relay 'IMb is deenerg'izedinliicating that the third trunk is idle, and the first trunk is also idle. In that case another circuit 'for' thes'tepping magnet is closed through the left hand arjnature and "back contact of relay 93; ithe second wiper of *allotter switch 8| in engagement with its third bankcontact', -conductor 'l li'4, the right hand inner armature and "front contact of relay 04a. conductor I I 5, the right hand miter armature and back contact or relay fil since the first trunk is'idle, and the right'hand-armature and back "contact-of relay I04 to ground. The additional relays I04 "associated with idle trunks are similarly connected 'to each other and to contacts 'of'the'switch'tll to effect the stepping oft he switch wipers back to "the position shown where the switch stops stepping and thereby preselects the'lowest numbered trunk for the next call; Similarlyif the first trunk was in use at the "moment and the second trunk was idle, the

" circuit of 'the stepping magnet ctOthB left hand outer armature and front contact of relay I04 or the right hand inner armature and front contact of relay 6| will cause wipers of the allotter switch to be stepped to their second bank contacts, even though the switch wipers are in engagement with the contacts of a higher numbered idle trunk, and the second trunk will be preselected.

If the answering trunk 2| is allotted to a call by the allotter switch 8I because of the fact that the previous lower numbered trunk or trunks are in use, the energization of relay 93 as described above subsequent to the initiating of the call from a branch office, will close a circuit through its right hand armature and front contact and the third wiper of the allotter switch 8| for operating relay 96a in the same manner as relay 96 is operated when the first trunk was available for the call. The energization of relay 96a renders the group control switch 80b operative in the same manner as described above. in connection with the group control switch 80 associated with the first group. Thereupon one of the finder switches 11, 11a would be rendered operative to connect the calling branch office to the switching center. The operation of the control relays associated with the trunk 2I would be effected in substantially the same manner as described above in connection with the control relavs associated with the trunk 20. Momentary interruption of the line circuit 2| would release relay Ma at the main office whereupon the printer II would be connected on the line circuit and relays 42a and I03a would be o erated andthe printer I I connected to the printer at the calling branch ofiice. At the end of the call the operation of the push button orrelease key I8a disconnects. the printer II from the line, extinguishes the supervisory lamp I la and causes the release of the switching equ ment at the switching center in substantially. the same man!- ner as described above in connection with the main ffice trunk 20.

In accordance with another feature of the invention a step-by-step switch 66 is provided to prevent faulty operation in response to failure of power at the switching center. feature are presented herein. this feature being claimed in a divisional application. Since it will be apparent that a call is initiated from each branch oiiice by deenergization of the normally energized relays 63; 63a. et cetera, and said relays when released close a short-circuit around their own windings. if there is even a momentary power failure resultingfrom power line failure or other causes. all of the relays 63, 63a,-et cetera would be released. When power was restored the switching equipment at the switching center would attempt to switch through calls from each of the branch offices of which there may be a considerable number. All of the trunks to the main oilice would be tied up until each call was answered and released in the usual manner. In order to avoid this difiiculty, a relay I25, Fig 4 is provided which is connected directly across the power terminals and is thus normally continually energized. When a power failure occurs, relay I25 becomes deenergized and closes'a short circuit from positive battery through aresistance I26, the inner armature and back contact of relay I 25, the second switch wiper. of the rotary.

switch, and the contacts of switch 81,.to prevent the relayfrom becoming energizedwhen power is restored to the switching equipment. A circuit is also closed upon the deenergization of relay I25 from ground through the outer @llllh? No claims to this 1 ture and back contact and the first wiper of switch 66 in engagement with its first bank contact for stepping said switch as soon as power is, restored. As soon as the wipers of switchIiB are stepped into engagement with their second bank contacts, relay I25 becomes energized and closes a stepping circuit for the switch through the first wiper and the remaining bank contacts engaged thereby and the outer armature and front contact of relay I25. .In this manner the wipers of switch 66 make one traverse as soon as power is restored to the equipment. The third wiper of switch 66 immediately opens the circuit through the first bank contact thereof which is multipled to the start circuit of the finder switch equipment, thus preventing placing of any calls until the switch returns to normal. Circuits are also closed through the second and third wipers of switch 66 and their respective bank contacts for momentarily operating relays'62, 62a, et cetera, thereby removing the short circuits around windings of associated relays 63, 6311, et cetera, and permitting said relays to be reenergized in series with the respective line circuits. In this manner the system is restored to an idle or inoperative condition in readiness for placing of subsequent calls from the branch oflices in the usual manner. 1

Various modifications of the embodiment of the invention specifically described above will occur to those skilled in the art and may be made without departing from the scopeof the invention as set forth in the appended claims.

We claim: 7 v

1. A multiofllce telegraph system comprising a main oflice, a plurality of branch oflices or stations, printing telegraph equipment in each of said ofiices or stations, a switching center remote from the main oflice for interconnecting said main ofiic'e with any desiredone of said branch ofiices or stations, a plurality of trunks from the main ofiice to the switching centercommon to all of said branch ofiices, line circuits at the switching centerto each of the branch oiiices,

means including selective switching equipment at said switching center for extending callsin either direction between the main. oflice'andany of the branch ofices and means for controlling said switching equipment to connect one of said trunks from the main oflice to a wanted branch oflice and at other times to connect the same trunk to any branch office which desires to transmit a message to the main ofiice.

2. A multiofi'ice telegraph system comprising a main ofiice, a plurality of branch oflices or sta tions, printing telegraph equipment in each 'of said oflices or stations, a switching center remote from the main omce containing switching equipment for interconnecting said main office with any desired oneof said branch 'ofiice or stations, line circuits from said switching center to each of said branch ofiices or stations, a, trunk from said main ofiice to said switching center; said switching equipment including a selector switch and a finder switch individual to said trunk,

means at the main office for directively controlling said selector switch over said trunk to connect said trunk to any desired branch ofiice or station and means at each branch oflice' or station for operating said finder switch to connect a calling branch office through said trunk to the main office when said trunk is idle. I

3. A multioffice telegraph system comprising a main ofiice, a plurality of branch ofilces or stations, printing telegraph equipmentin each of said offices or stations, 9, switching center remote from the main office containing switching equipment for interconnecting said main oflice with any desired one of said branch ofiices or stations, line circuits from said switching center to each of said branch oilices or stations, said switching equipment including a finder switch to select the line circuit of a calling branch oifice or station, means at each branch ofiice or station for initiating a call to the main office and thereby operating the finder switch to connect the calling branch oilice or station to the main ofiice and means controlled from the main Office to render said finder switch inoperative and select a wanted branch office in the case of a call from the main oilice to said wanted branch oifice.

4. A telegraph exchange system comprising a main office, a branch oflice, a trunk terminating at the main office, printing telegraph apparatus at said ofllces arranged to employ said trunk for communication, a source of current for said trunk at the main ofiice, means at the branch oflice for momentarily interrupting the flow of current in said trunk toinitiate a call to the main ofiice and means including a relay in said trunk circuit at the main oflice responsive to said momentary interruption for reversing at its contacts the polarity of the source of current connected to said trunk.

5. A telegraph exchange system comprising a main oflice, a branch oflice, a trunk terminating at the main office, a source of current for said trunk at the main oflice, switching means for momentarily interrupting the flow of current in said trunk upon initiating a call from the branch oflice to the main oilice, means including a relay at the main oflice responsive to said momentary interruption for reversing the polarity of the source of current connected to said trunk, means for operating said switching means and means associated with said switching means responsive to restoration of the trunk to its normal or idle polarity for releasing said switching means at the end of the call.

6. A telegraph exchange system comprising two offices, a trunk terminating at one office, means for momentarily interrupting the flow of current in said trunk to initiate a call to said one oflice, means including a relay at said one oflice responsive to said momentary interruption for reversing the polarity of the trunk, switchin means at said other office associated with said trunk and means for controlling said switching means in response to restoration of the trunk to its normal or idle polarity at the end of the call.

7. A telegraph system comprisin -a line circuit, a normally energized relay in series relation with said line circuit, means whereby said relay remainsdeenergized after the circuit thereof has been interrupted by a momentary opening of the line circuit and means including the contacts of said relay for directly controlling the polarity of said trunk.

8. A telegraph exchange system comprising two ofiices, a trunk terminating at one oillce, means for momentarily interrupting the flow of current in said trunk to initiate a call to said one omce, means including a relay normally in said trunk circuit at said one ofiice responsive to said momentary interruption for reversing at its contacts the polarity of the trunk and means at the other oflice for detecting the reversal of polarity of the trunk.

9. A telegraph exchange system comprising two oflices, a trunk connecting said oflices, a path including said trunk which is normally conductive during idle periods, means for momentarily interruptin the flow of current in said trunk to initiate a call to one of said offices from the other office, switching means at said other oflice associated with said trunk and means at said one oilice responsive to said momentary interruption of the flow of current in said trunk for controlling said switching means.

10. A telegraph exchange system comprising a main office, a branch ofiice, a trlmk terminating at the main office, printers adapted to be interconnected through saidtrunk, means including a source of current at the main ofiice for impressing potential of one polarity on said trunk during idle periods, means at the branch office for momentarily interrupting the flow of current in said trunk to initiate a call to the main oflice and means including a relay normally in said trunk circuit at the main oflice responsive to said momentary interruption for changing at its contacts the character of the potential connected to said trunk to indicate the seizure thereof for communication.

FRANK LESLIE CURRIE. CHARLES F. STEARNS. 

